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FM31276 Datasheet(PDF) 7 Page - Ramtron International Corporation |
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FM31276 Datasheet(HTML) 7 Page - Ramtron International Corporation |
7 / 26 page FM31278/276/274/272 - 5V I2C Companion Rev. 2.1 Sept. 2011 Page 7 of 26 to be loaded into the timekeeper core. W is used for writing new time values. Users should be certain not to load invalid values, such as FFh, to the timekeeping registers. Updates to the timekeeping core occur continuously except when locked. Backup Power The real-time clock/calendar is intended to be permanently powered. When the primary system power fails, the voltage on the VDD pin will drop. When VDD is less 2.5V the RTC (and event counters) will switch to the backup power supply on VBAK. The clock operates at extremely low current in order to maximize battery or capacitor life. However, one of the advantages of combining a clock function with the F-RAM memory is that data is not lost regardless of the backup power source. The IBAK current varies with temperature and voltage (see DC parametric table). The following graph shows IBAK as a function of VBAK. These curves are useful for calculating backup time when a capacitor is used as the VBAK source. Figure 7. IBAK vs. VBAK Voltage The minimum VBAK voltage varies linearly with temperature. The user can expect the minimum VBAK voltage to be 1.23V at +85°C and 1.90V at -40°C. The tested limit is 1.55V at +25°C. The minimum VBAK voltage has been characterized at -40°C and +85°C but is not 100% tested. Figure 8. VBAK (min.) vs. Temperature Trickle Charger To facilitate capacitor backup, the VBAK pin can optionally provide a trickle charge current. When the VBC bit, register 0Bh bit 2, is set to „1‟, the V BAK pin will source approximately 80 µA until VBAK reaches 3.75V. In 5V systems, this charges the capacitor to VDD without an external diode and resistor charger and also prevents the user from exceeding the VBAK maximum voltage specification. There is a Fast Charge mode which is enabled by the FC bit (register 0Bh, bit 5). In this mode the trickle charger current is set to approximately 1 mA, allowing a large backup capacitor to charge more quickly. In the case where no backup source is used, the VBAK pin should be tied to VSS. VBAK should not be tied to 5V since the VBAK (max) specification will be exceeded. Be sure to turn off the trickle charger (VBC=0), otherwise charger current will be shunted to ground from VDD. Note: systems using lithium batteries should clear the VBC bit to 0 to prevent battery charging. The VBAK circuitry includes an internal 1 K series resistor as a safety element. The trickle charger is UL Recognized. |
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